How do time zones and daylight saving time actually work?

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It is midday in London, evening in Seoul and early morning in New York, all at the same moment. And twice a year, in many countries, everyone's clocks jump by an hour. Here is how the system works and how it came about.

Why we need time zones at all

For most of history, towns kept their own local time based on the Sun. Noon was when the Sun was highest in the sky, which meant that a town to the west was a few minutes behind a town to the east. Nobody minded much while travel was slow. Railways changed that. A timetable is impossible if every station runs on a different clock. In Britain, railway companies began using Greenwich Mean Time in the 1840s, and in 1847 the Railway Clearing House recommended it for all lines. GMT became the legal time for Great Britain in 1880. In 1884 an international conference in Washington chose the meridian through Greenwich as the starting line for world longitude and time.

How the zones are laid out

The Earth turns 360 degrees in 24 hours, which is 15 degrees every hour. In theory, then, the world divides into 24 zones about 15 degrees wide, each an hour apart. In practice the borders bend and wiggle to follow countries and regions. Today the reference is Coordinated Universal Time (UTC), kept by atomic clocks. Every zone is described as an offset from it, such as UTC+9 or UTC-5. A few quirks:

Not every offset is a whole hour. India uses UTC+5:30 and Nepal UTC+5:45.
Big countries choose differently. The United States and Russia span several zones, while China uses a single time across the whole country.
The International Date Line in the Pacific is where the calendar date changes. Cross it heading west and you skip ahead a day.

What daylight saving time is

Daylight saving time means moving clocks forward an hour in spring and back in autumn, so that more of the daylight falls in the evening when most people are awake. In the UK, the idea was championed by the builder William Willett in the early 1900s. It was adopted in 1916 during the First World War to save fuel, and the summer period is now called British Summer Time (BST), which is GMT plus one hour.

The UK's recurring pattern

Clocks go forward one hour at 1am on the last Sunday in March. You lose an hour of sleep.
Clocks go back one hour at 2am on the last Sunday in October, returning to GMT. You gain an hour.

A handy way to remember: spring forward, fall back. Phones and computers change automatically. Ovens, car clocks and older watches usually do not.

Not everyone does it

Countries near the equator gain little, because day length barely changes through the year.
Much of Asia, including South Korea, Japan and China, does not change its clocks.
Countries that do change may switch on different dates, so the gap between, say, London and New York can be an hour different from usual for a couple of weeks each year.
In the Southern Hemisphere, summer time runs across the northern winter.

The arguments continue. Supporters point to lighter evenings for leisure and shopping. Critics point to disrupted sleep after the change and darker mornings in winter. Proposals to stop changing the clocks come up regularly in many countries.

Tips for living with time zones

Write meeting times with the zone, for example 15:00 UK time, or use UTC for international groups.
Use a world clock on your phone to keep track of friends and colleagues abroad.
Check both ends around the switch dates, since a weekly call can shift by an hour for a while.
Remember the date, not just the hour, when calling somewhere far ahead or behind.

Which time zone confusion has caught you out? Share your story in the comments.